Generally, tapping screws are threaded into workpieces made of soft materials such as resin, aluminum alloys, etc. A representative example of such tapping screws was proposed in Japanese Patent No. 3290362. In this tapping screw, an external thread is spirally formed on an outer circumferential surface of a shaft part. By virtue of the external thread, the tapping screw forms an internal thread in a hole formed in a workpiece while the tapping screw is threaded into the hole. Furthermore, portions of a ridge of the external thread are cut out in the same direction as that of an axis of the tapping screw so as to form clearance grooves. Portions of the workpiece that are compressed by the external thread when the internal thread is formed, are embedded into the clearance grooves, thus exerting an anti-loosening effect.